Idle cut-off and oddball Magnetos
There are a number of posts in this thread that triggered thoughts in me.
First - Idle cut-off. Yes we typically shutdown by shutting off the fuel supply in the carburettor. I’ve always understood this is done for two reasons;
1) Safety - Remove the fuel from the intake so there is nothing to burn. More on this in a sec
2) Create a lean condition where any possible hot-spots in a cylinder can’t ignite the remaining air/fuel after shutdown leading to the engine running on even when the magnetos/ignition are off (dieseling).
Now as for no.1 above. I grew up around vintage aircraft. When I was a kid I was thoroughly educated by my father about the danger of props. It was probably about the time that golfer walked into a prop, when I remember Dad demonstrating how you could never trust the engine not to start. He shutdown the Franklin engine on his Stinson with the idle cut-off. He then explained what that meant and why we did it. He then left the mixture at idle-cutoff, turned the Mags on and cranked the engine. It fired up and ran a few blades. - Certainly enough to drive home the point he was making.
Secondly; Another one of teachable moments came not long after I got my first aeroplane flying, A British built 1946 Auster J4 with an inverted inline Cirrus Minor, with no electric starter, and British BTH Magnetos. I’d checked, gapped the points and replaced the points cap and it’s P-lead. When I started the engine next, by hand-propping, while ‘sucking in’, (priming the cylinders after flooding the intake manifold for a cold start), The engine ‘popped’ or fired as it went through compression and the impulse let go. It didn’t run just popped enough to really get my attention. I checked the mag toggles were off, checked the wiring and earthing all were good and not faulty. I then went on to start the engine normally with the impulse Mag on (the right Mag on this aeroplane). I then double checked each mag shutdown the engine. No problems were obvious.
A little while later it dawned on me. The British BTH SG-4 mags earth the points through the mag cap with a carbon brush connected to the P-lead that rubs on the centre shaft of the points cam. The body of the points are earthed and the centre shaft is insulated and you earth the centre shaft via the mag cap and this carbon brush. A strangely complicated way of doing things, but typically British. In servicing the point gaps I’d either introduced a speck of dirt or moved the carbon brush enough that it didn’t properly disable one of the Mags.
Now before removing the plug points cap, I remove all the plugs and plug leads before touching anything on the Mags. You need to know an individual aircraft’s systems and design properly to know its failure modes.
And as far as the Auster carburettor goes, while it has a mixture control it doesn’t have an idle cut-off - Much like the Stromberg carburettors that are typically on Continental A-65’s and C-85’s etc. So my standard shutdown here is to idle, (500-600RPM) the engine for 30-60 seconds, which helps cool down the cylinder heads, turn both mags off and as the prop slows below an idle open the throttle wide to help create a lean condition where I don’t get dieseling.
Over the years of flying together Dad & I had plenty of learning experiences that involved failed mags, intermittent failures and broken P-leads etc. Any time I move a prop I expect the engine to fire. I never move the prop ‘just because’. I never lean on props and don’t allow anyone near me to do so either. If I have to move the prop for servicing etc. all plug leads are removed/disconnected and most times both set’s of plugs are out too unless the other plug has to be in the cylinder for differential compression tests.
And to keep this vaguely RV related, on our RV-6, I check each mag at 1000RPM by turning the [key]switch to R then L, confirm I get a RPM drop on each then to completely off and back on before pulling the mixture to idle cut-off.
I will be using the mag earth screw and an Adel cushion clamp to support the P-lead as DanH showed above. Such a brilliantly simple idea. Thank you DanH
Don H
1946 AusterJ4
1998 RV-6